Filariasis is a parasitic disease that is caused by thread-like filarial nematodes or roundworms. Filariasis is a vector-borne disease that is transmitted via insect bites. Infective larvae of the nematodes can be introduced into the human body via bites of blood sucking insects such as mosquitoes or flies.
Filariasis can also affect domestic animals such as dogs. In dogs, dirofilariasis which is also called heartworm disease, is caused by nematodes called Dirofilaria immitis and Dirofilaria repens. Dirofilariasis is considered endemic in 49 States of the United States. The vectors are also blood sucking insects such as mosquitoes.
The major causes of human filariasis are the filarial nematodes Wuchereria bancrofti, Brugia malayi, Brugia timori, Onchocerca volvulus and Mansonella species that have human hosts. The nematodes Wuchereria bancrofti, Brugia malayi and Onchocerca volvulus are responsible for most of the debilitating filarial infections in more than 80 developing countries of the tropics and sub-tropics where 1.1 billion are at risk of infection and about 150 million are infected. All three species are a source of severe pathologies that result in high morbidity and increased mortality. The infection can cause severe morbidity in up to 50% of those infected with the nematodes.
W. bancrofti and B. malayi infections can develop into lymphatic filariasis, often seen as hydrocoele in men and/or lymphedema, and in extreme cases, elephantiasis. O. volvulus infections can develop into severe dermatitis and/or onchocerciasis, the visual impairment giving the latter disease its common name River Blindness. Community-directed mass drug administration programs are designed to control these infections and eliminate them as a public health problem.
Current efforts aim to eliminate these parasitic nematodes through the use of drugs like diethylcarbamazine, ivermectin, and albendazole that kill the larvae, but not the adult worms. The antihelmintic drug diethylcarbamazine is used to combat lymphatic filariasis in countries without co-endemic O. volvulus infections, i.e., outside of Africa. Ivermectin is used to combat onchocerciasis. The greatest efficacy of both drugs is against the first stage larvae found in the blood stream or in the dermis. Since the worms can live up to 14 years and are fecund for most of their lifespan, populations in endemic regions must be treated with high coverage (at least 65%) for many years to break the transmission of the disease to uninfected persons. The presence of larvae in the skin or blood of individuals, especially children, even after many rounds of treatment additionally demonstrates that non-responders exist to the current anti-filarial drugs. There are further growing concerns that resistance in the worms may be developing. Because diethylcarbamazine can lead to severe adverse reactions in O. volvulus infected persons, ivermectin is effectively the sole drug available for controlling onchocerciasis.
The nematodes are host to obligate intracellular bacteria of the genus Wolbachia. These endobacteria are essential for embryogenesis, larval development and adult worm survival. An alternative effort aiming to eliminate the nematodes is the removal of the bacteria with antibacterial agents. While hundreds of antibiotic compounds having highly different chemical structure have been identified, it is nearly impossible to choose which compound to use in treating a specific disease.